University of Michigan - Department of Astronomy

Version: 105

The Milky Way Disk

Continuous as the stars that shine
And twinkle on the Milky Way

--William Wordsworth, "Daffodils"


Introductionluminosity classes on the Hertzsprung-Russell diagram

Luminosity Classes and Stellar Evolution

Figure 1: Luminosity Classes on the Hertzsprung-Russell diagram

You know that stellar spectral types correspond to the stellar surface temperatures, which are plotted on the x-axis of the H-R Diagram. O stars are the hottest and bluest, M stars the coolest and reddest, with G stars like the Sun at intermediate temperatures.  Astronomers also have a system for specifying luminosity classes, which correspond to stellar luminosities. These are a rougher classification scheme than the spectral types, and are more approximately represented on the y-axis of the H-R Diagram (Figure 1).

A star's luminosity correlates with its radius. Given two stars with the same temperature, the bigger one will be more luminous. Supergiants like Betelgeuse are bigger than the Earth's orbit!

The luminosity classes are specified with Roman numerals:

• Classes I and II: Supergiants*

• Class III: Giants

• Class IV: Subgiants

• Class V: Dwarfs

*Note Class II stars are technically called "bright giants", but they are also massive stars, like supergiants.

The Sun is a dwarf star, so it is a G V star. All Main Sequence stars are dwarfs (class V).

The luminosity classes are associated with different age stellar populations, because most stars leave the Main Sequence and increase in luminosity near the ends of their lives.  Stars just leaving the Main Sequence are subgiants (class IV), and red giants (class III) have clearly left the Main Sequence. Thus, both luminosity classes III and IV are associated with older stellar populations. On the other hand, massive stars maintain the same, extremely high luminosity, for most of their short lives. Therefore, supergiants (classes I and II) are all massive stars and are associated with young populations. Betelgeuse, in the constellation Orion, is clearly red even to the naked eye, and it is an example of a short-lived, massive star that has become a red supergiant. Note that all of the red stars visible to the naked eye are red giants (class III) or red supergiants (class I), because red dwarfs (class V) are too faint to be seen. Note that only normal stars undergoing nuclear fusion are assigned luminosity classes. For example, white dwarfs and neutron stars do not have a luminosity class.

Are all types of stars found in equal numbers everywhere in our Milky Way Galaxy? Stars have a tendency to migrate away from their birth location in the central plane of the Milky Way disk. The entire disk is rotating at 220 km/s, and as the stars travel throughout their lives, they jostle gravitationally with other stars and objects. This causes older stars to be less confined to the plane than younger stars. Thus, the youngest populations most closely hug the plane of the Milky Way, while the oldest are found in a more "puffed up" distribution around the plane. In this activity, you will see how different stellar luminosity classes trace different age populations in the Milky Way disk.


The Milky Way Disk: Worksheet

Part 1: Luminosity Classes and the Milky Way

  1. Your GSI will hand out materials to four groups. Each group will receive a list of bright stars belonging to one of the luminosity classes, a star chart, and a laser pointer. Note the luminosity class of your stars here:

  2. Mark the stars on your star chart so you can easily find them in the sky. Each person in the group should be responsible for locating at least one star. It may help to circle the stars' constellations, too.

  3. The GSI will set the sky to a level where its surface brightness is fairly high, but where the brighter stars can be seen. The GSI will ask each group to identify their stars, by luminosity class. Try to identify which luminosity class best traces the Milky Way, and which is the worst.

  4. Once all the groups have identified their stars, your GSI will turn the lights down so you can see the Milky Way. Which luminosity class best traces the Milky Way? 

    Which is the worst?

  5. Molecular clouds and star-forming regions are also associated with the Milky Way disk. The GSI will point out some of these. How do they compare as a tracer of the Milky Way?

Concluding Questions

  1. The accompanying list of stars shows two columns, Galactic longitude and Galactic latitude, l and b, respectively. Plot the positions of the luminosity class I and II stars using their Galactic longitude and latitude. Using a different symbol or color, plot the positions of the luminosity class III stars on the same graph.  Repeat for class V.  You should end up with one graph with three different symbols or colors. You can make the plot by hand on graph paper, or use any kind of software. Based on your graph, which luminosity class best traces the Milky Way and which is the worst? Explain why.

  2. You plotted these same stars in equatorial coordinates on the star charts. Reviewing that work, can you determine the locus of the central plane of the Milky Way in equatorial coordinates? There are different ways to answer this question, so please discuss your answer clearly.

  3. Besides their luminosity class and position, name another property of these bright stars that would be useful for confirming that we live in a flat disk-like structure. Explain.

  4. Your list of stars shows the brightest stars in the sky. Why do they belong to all different luminosity classes?

  5. Which luminosity class of stars (I, III, or V) would have a similar distribution on the sky as white dwarf stars? As core-collapse sueprnovae? Explain your answers.

Last modified: 08/27/14 by MSO

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